Try breathing through a soda straw. It takes effort, and you'll probably get dizzy soon. A fuel-burning boiler, furnace, or water heater can also end up gasping for air when installed in a very tight house or a confined space. The appliance won't get dizzy, but it can't burn its fuel properly -- resulting in reduced efficiency, and possibly exposing occupants to harmful gases such as carbon monoxide. A fuel-burning appliance needs a lot of air to burn properly. For example, a 100,000 Btu/h boiler or furnace needs 1,250 cubic feet of air per hour for proper combustion. With the exception of vent-free gas fireplaces and kitchen ranges, all gas- and oil-fired appliances are vented, using either fan-assisted venting or traditional atmospheric venting via a chimney.
Thanks to the natural buoyancy of hot flue gas -- which rises rapidly up the vent pipe, just as a hot air balloon rises in the open air -- vent pipes and chimneys suck flue gas out of appliances and suck in additional air to replace it. Here's the rub: You must replace the same amount of air that rises up the vent, and the makeup air must flow...
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